This invention relates generally to optical resonators. More particularly, it relates to optical frequency converters.
Optical resonators find use in a variety of laser based optical devices, such as frequency doublers, optical parametric oscillators, mode cleaners, frequency filters, temporal filters and spatial filters. An optical resonator generally comprises two or more mirrors configured to reflect light back and forth over an optical path or around a loop. Light from an external (or pump) light source, such as a laser, builds up a large signal within the resonator when the optical path length matches an integral number of wavelengths of the source light. In order to achieve highly efficient frequency conversion, external resonators can be used to build-up high circulating powers at the pump frequency. Similarly, light can build up from noise when an internal source of optical gain (such as a laser medium or parametric amplifier) is contained within an optical resonator.
Resonators find application in optical parametric oscillators (OPO""s). An OPO is a nonlinear device that converts incident photons into photon pairs when optically excited at a power per unit area above a certain threshold. The threshold level is a characteristic of the non-linear material and the resonator. Suitable non-linear materials include Lithium Niobate (LiNbO3), Lithium Tantalate (LiTaO3), Lithium Borate (LiBO3) PPLN, PPLT MgO:PPLN, KTP, PPKTP, RTA, BBO, PPRTA, and the like. OPO""s are usually embodied in one of two forms: Either a doubly resonant oscillator (DRO) in which both the generated optical beams are resonated or in a singly resonant oscillator (SRO) in which only one of the generated optical beams is in resonance. Furthermore, non-planar ring oscillator (NPRO)-based pump-resonant OPO""s have been demonstrated in several wavelength ranges. (see, e.g., K. Schneider and S. Schiller, xe2x80x9cNarrow-linewidth, pump-enhanced singly-resonant parametric oscillator pumped at 532 nmxe2x80x9d, Applied Physics B 65, 775, (1997), and K. Schneider, P. Kramper, S. Schiller, and J. Mlynek, xe2x80x9cToward an Optical Synthesizer: A Single Frequency Parametric Oscillator Using PPLNxe2x80x9d, Opt. Lett. 22, 1293, (1997), and D. Chen, D. Hinkley, J. Pyo, J. Swenson, and R. Fields, xe2x80x9cSingle-Frequency, Low-threshold continuous-wave 3-xcexcm Periodically Poled Lithium Niobate Optical Parametric Oscillator.xe2x80x9d)
If the optical path within the resonator changes by a substantial amount (about one wavelength), then the OPO will mode-hop. In order to prevent this, prior art systems have included means to control total optical path length within the cavity. These means often involve controlling the index of refraction of a transmissive material in the optical path. Such means typically change the length of the optical path by changing a temperature of the non-linear element. Unfortunately, changing the temperature of a non-linear material also changes the frequency of maximum optical gain, which is undesirable because this can also induce mode hops. Furthermore, many applications of resonators, such as mode cleaners, frequency filters, temporal filters and spatial filters, do not use a non-linear optical element, or any other intracavity transmissive element.
Alternatively, the optical path length may be changed by moving one of the mirrors with respect to the rest of the optical cavity, e.g. with a piezoelectric element. Typical resonator configurations include discrete, monolithic, semi-monolithic, and quasi-monolithic resonators. In a discrete resonator two or more mirrors are independently mounted to a support structure such as an optical table. The length of the optical path in a discrete resonator can be readily adjusted to tune the resonant frequency. Unfortunately, discrete resonators are subject to variations in the length of the optical path due to mechanical instability of the mirror mounts.
A monolithic resonator comprises a single block of transparent material having reflecting facets that serve as the mirrors. Usually, the material is strained by changing its temperature. Alternatively, the optical path length of a monolithic oscillator can be adjusted by a piezoelectric element mounted to uniformly strain the entire block in a plane parallel to the plane of the optical path. Such a configuration is described in U.S. Pat. No. 4,829,532 issued May 9, 1989 to Kane and assigned to the assignee of the present application. The monolithic resonator is extremely rugged and resistant to mechanical perturbation. Unfortunately the monolithic resonator is difficult to manufacture. Furthermore, piezoelectric elements provide only a limited range of motion to the block and, therefore, only a limited range of tuning of the resonator frequency.
Semi-monolithic resonators are often used in OPO""s. In a semi-monolithic OPO, a block of non-linear material having at least one reflecting facet is mounted separate from a mirror to form a resonator. The resonator is tuned by adjusting the distance between the mirror and the block of non-linear optical material. The optical pump within the resonator couples with the non-linear optical material, which produces an output signal as a result. This arrangement is more adjustable than the monolithic resonator and more rugged than the discrete resonator. Unfortunately, the semi-monolithic resonator has two extra surfaces that introduce losses and is difficult to fabricate.
Quasi-monolithic resonators are described in N. Uehara, E. K. Gustafson, M. M. Fejer and R. L. Byer in xe2x80x9cModeling of Efficient Mode Matching and Thermal-Lensing Effect on a Laser-Beam Coupling Into a Mode Cavity Cleanerxe2x80x9d, Proceedings of the International Society for Optical Engineering vol 2989, p. 57-67. A typical quasi-monolithic optical resonator 100 of the prior art is depicted in FIG. 1. The resonator generally comprises a monolithic structure 101, three mirrors M1, M2, M3, and a piezoelectric element 104. Structure 101 is made from a single block of a low thermal expansion material, such as Zerodur(trademark). Holes drilled in the block form an optical path 110. Mirrors M1, M2, M3, are fixed to the structure 101 and configured to deflect light back and forth along optical path 110. A large optical signal builds up within structure 101 when a length of optical path 110 matches an integral number of wavelengths of light from a source such as a laser. To tune structure 101 one of the mirrors, e.g., mirror M3, is mounted to piezoelectric element 104, which is mounted to structure 101. A voltage applied to piezoelectric element 104 moves mirror M3 by a small amount. This method works well enough for rapid mirror movements of about 1 micron or less. However, for long range adjustment, e.g., about 1 to 10 microns or more, several piezoelectric elements must be stacked together or a large voltage, e.g. greater than 100 volts, must be applied. The resulting structure tends to be long, fragile and unstable.
There is a need, therefore, for a robust optical resonator that is continuously and finely tunable over a broad range.
Accordingly, it is a primary object of the present invention to provide a mechanically robust optical resonator capable of continuous and fast tuning over a broad range of wavelengths. It is a further object of the invention to incorporate such a resonator into an optical parametric oscillator.
These objects and advantages are attained by a broadly tunable quasi-monolithic optical resonator. The resonator comprises a piezoelectric element attached to a quasi-monolithic structure. The quasi-monolithic structure defines an optical path. The optical path defines a cavity. At least two mirrors attached to the structure are configured to deflect light along the optical path. The piezoelectric element controllably strains the quasi-monolithic structure to change a length of the optical path by at least about 0.01 micron. A first feedback loop coupled to the piezoelectric element provides xe2x80x9cfinexe2x80x9d control over the cavity length. A preferred embodiment of the resonator includes a thermally actuated spacer attached to the structure and second mirror attached to the spacer. The thermally actuated spacer adjusts the cavity length by up to about 20 microns. The spacer typically includes a thermally responsive block, a heater element and perhaps a temperature sensor. A second feedback loop coupled to the sensor and heater provides a xe2x80x9ccoarsexe2x80x9d control over the cavity length.
Another embodiment of the invention provides a quasi-monolithic optical parametric oscillator (OPO). This embodiment includes a non-linear optical element along an optical path within a quasi-monolithic resonator structure. The structure is strained by a piezoelectric element for fine control and one mirror is mounted to a thermally actuated spacer for coarse control. The non-linear element is typically temperature controlled. Such an OPO configuration is broadly tunable and capable of mode-hop free operation for periods of 24 hours or more.